The present invention relates to a golf club head having a hollow behind the face portion, more particularly to a structure of the face portion provided with a specific thickness distribution.
Recently, there have been widely used wood-type hollow golf club heads whose face portion is made of a thin metal material in order to increase carry and reduce the weight of the face portion, while providing a thicker central region to provide necessary strength and durability.
In this structure, since the central region is surrounded by the relatively thin region, there is a tendency that a large stress at impact tends to concentrate on the upper edge and lower edge of the face portion (namely, annular thin region) because, in comparison with the toe-side edge and heel-side edge, the upper and lower edges are approximate to the central region which is moved largely at impact.
United States Patent Application Publication No. US-2003-144079-A1 discloses a wood-type golf club head, wherein, in order to increase the strength of the face portion, the thin region surrounding a thick central region is provided with narrow ribs extending radially from the central thick region.
Japanese Patent Application Publication No. P2006-175135A  discloses a golf club head whose face portion is provided with a thicker part in order to minimize the difference in carry distance between sweet spot shots and off-center shots towards the heel and toe, wherein the thicker part is disclosed as having a thickness of 4 mm to 10 mm and extending almost parallel to the vertical direction with a substantially constant width, and the resultant thin part of the face portion is disclosed as having a thickness of less than 2.5 mm.
On the other hand, according to a research made by the present inventor et al., as shown in FIG. 3, hitting positions of average golfers have a tendency to distribute along a straight line N extending between the toe and heel at an inclination angle of about 20 to 30 degrees with respect to the horizontal direction.
Therefore, in view of such distribution, there is plenty of scope for minimizing the above-mentioned difference in the carry distance and improving the strength and durability of the face portion, while the reduced overall thickness of the face portion is maintained, and a large variation of the thickness is restrained for easier manufacturing.